The present invention relates to a method for stimulating a subterranean zone through the deferred injection of fluid under pressure coming from a neighbouring zone by means of a deflected drain running through a slightly permeable intermediate layer. What is called a deflected drain hereinafter is any wellbore at least part of which is horizontal or relatively slightly inclined in relation to the horizontal.
The method according to the invention more particularly stimulates the production of a petroliferous zone separated from an underlying zone containing a fluid under pressure, such as an aquiferous zone or possibly a petroliferous zone.
Various techniques known by specialists are utilized for stimulating the production of petroliferous zones. One of them essentially consists in injecting a fluid under pressure into the formation in production, capable of draining the oil stagnating in the rocks because of its viscosity. The fluid used is, for example, water under pressure injected by drains bored through the formation. It can also be water existing in depth, in the basin in production itself, in the form of an underlying aquiferous nappe.
In certain types of basins, the aquiferous nappe lies under the petroliferous zone and is separated from the zone by a layer which is slightly permeable notably because of the presence of heavy and very viscous hydrocarbon products (tarmat).
It can be envisaged to utilize this underlying water to stimulate the production of a petroliferous zone. The water nappe being located at a depth greater than that of the petroliferous zone, its inner pressure is higher. The at least partial depletion of the zone in production has the effect of increasing the overpressure of the water in the underlying nappe in relation to the fluids in the zone above. The possible injection into the petroliferous formation of this overpressure water should act to drain the oil and to stimulate production.
The attempts to stimulate production zones topping aquiferous nappes have not yet produced the expected results. Vertical wells or drains have been bored through the petroliferous zone in order to make the petroliferous deposit communicate with the water nappe. But it has been noticed that this type of well essentially produces water. This negative result can be explained by the fact that the water in the nappe tends to directly escape towards the surface through the well that has been created, instead of entering the petroliferous formation. This phenomenon persists if an obturation device is taken down into the well, because the water of the nappe tends to flow round it through the surrounding formations. A possible obturation near the surface causes a certain diffusion of the water in the petroliferous zone. But the results are not very significant because the volume of the deposit penetrated by the water remains relatively low.